N-Phosphonomethylglycine (“glyphosate”) is a well-known, post-emergent, foliar-applied herbicide. When glyphosate formulations are applied to green leaves or stems, glyphosate moves through the plant so the entire plant dies. Glyphosate works by disrupting a plant enzyme, EPSP synthase, involved in the production of amino acids that are essential to plant growth. Because the enzyme is not present in humans or animals, glyphosate has very low toxicity to humans or animals. Glyphosate is typically formulated and applied in the form of a water-soluble agriculturally acceptable salt, e.g., the potassium or isopropylamine salt.
Surfactants are employed as adjuvants in glyphosate salt formulations to enhance herbicidal effectiveness. Surfactants help the formulations perform by improving two aspects of a spray droplet: wetting of the leaf surface and penentration of the spray droplet through the leaf cuticle into the plant tissues. Adjuvants further aid in the systemic translocation of the herbicide throughout the plant. A wide variety of surfactants have been taught as suitable for use (see, for example, the exhaustive lists of surfactant types taught in U.S. Pat. Nos. 7,049,270 and 7,135,437). A continuing issue with glyphosate formulations is how to formulate highly concentrated aqueous glyphosate compositions while maintaining good stability, particularly at cold or elevated temperatures (see, e.g., U.S. Pat. Appl. Publ. No. 2009/0318294).
Glyphosate formulations have been prepared containing surfactants such as fatty amine oxides, amidoamines, alkylbetaines, and other compositions (see, e.g., U.S. Pat. Nos. 6,908,882 and 6,992,046 and U.S. Pat. Appl. Publ. Nos. 2010/0113274 and 2009/0018018). These surfactants are made by derivatizing fatty esters or amides made from triglycerides, typically animal or vegetable fats. Consequently, the fatty portion of the acid or ester will typically have 6-22 carbons with a mixture of saturated and internally unsaturated chains. Depending on source, the fatty acid or ester often has a preponderance of C16 to C22 component. For instance, methanolysis of soybean oil provides the saturated methyl esters of palmitic (C16) and stearic (C18) acids and the unsaturated methyl esters of oleic (C18 mono-unsaturated), linoleic (C18 di-unsaturated), and α-linolenic (C18 tri-unsaturated) acids. The fatty ester is a good starting material for making the amine oxide, amidoamine, or alkylbetaine.
Recent improvements in metathesis catalysts (see J. C. Mol, Green Chem. 4 (2002) 5) provide an opportunity to generate reduced chain length, monounsaturated to feedstocks, which are valuable for making detergents and surfactants, from C16 to C22-rich natural oils such as soybean oil or palm oil. Soybean oil and palm oil can be more economical than, for example, coconut oil, which is a traditional starting material for making detergents. As Professor Mol explains, metathesis relies on conversion of olefins into new products by rupture and reformation of carbon-carbon double bonds mediated by transition metal carbene complexes. Self-metathesis of an unsaturated fatty ester can provide an equilibrium mixture of starting material, an internally unsaturated hydrocarbon, and an unsaturated diester. For instance, methyl oleate (methyl cis-9-octadecenoate) is partially converted to 9-octadecene and dimethyl 9-octadecene-1,18-dioate, with both products consisting predominantly of the trans-isomer. Metathesis effectively isomerizes the cis-double bond of methyl oleate to give an equilibrium mixture of cis- and trans-isomers in both the “unconverted” starting material and the metathesis products, with the trans-isomers predominating.
Cross-metathesis of unsaturated fatty esters with olefins generates new olefins and new unsaturated esters that can have reduced chain length and that may be difficult to make otherwise. For instance, cross-metathesis of methyl oleate and 3-hexene provides 3-dodecene and methyl 9-dodecenoate (see also U.S. Pat. No. 4,545,941). Terminal olefins are particularly desirable synthetic targets, and Elevance Renewable Sciences, Inc. recently described an improved way to prepare them by cross-metathesis of an internal olefin and an α-olefin in the presence of a ruthenium alkylidene catalyst (see U.S. Pat. Appl. Publ. No. 2010/0145086). A variety of cross-metathesis reactions involving an α-olefin and an unsaturated fatty ester (as the internal olefin source) are described. Thus, for example, reaction of soybean oil with propylene followed by hydrolysis gives, among other things, 1-decene, 2-undecenes, 9-decenoic acid, and 9-undecenoic acid. Despite the availability (from cross-metathesis of natural oils and olefins) of unsaturated fatty esters having reduced chain length and/or predominantly trans-configuration of the unsaturation, surfactants have generally not been made from these feedstocks.
We recently described new compositions made from feedstocks based on self-metathesis of natural oils or cross-metathesis of natural oils and olefins. In particular, we identified esteramines and ester quats, fatty amides, fatty amines and amidoamines, quaternized amines, betaines, sulfobetaines, alkoxylates, sulfonates, sulfo-estolides, and other compositions made by derivatizing the unique feedstocks (see application Ser. Nos. 13/878,550, 13/878,556, 13/878,972, 13/878,981, 13/879,786, and 13/880,007, respectively), all filed Oct. 25, 2011. The feedstocks, which include metathesis-derived C10-C17 monounsaturated acids, octadecene-1,18-dioic acid, and their ester derivatives, preferably have at least 1 mole % of trans-Δ9 unsaturation. Because the ability of a particular surfactant or blend of surfactants to impart stability to a glyphosate formulation over a wide temperature range is not easily inferred from surfactant structure, we performed extensive experimental investigations to identify subclasses of surfactants having these benefits.
New surfactant classes are always of interest to formulators of glyphosate formulations. Surfactants based on renewable resources will continue to be in demand as alternatives to petroleum-based surfactants. Traditional natural sources of fatty acids and esters used for making surfactants generally have predominantly (or exclusively) cis-isomers and lack relatively short-chain (e.g., C10 or C12) unsaturated Zs fatty portions. Metathesis chemistry provides an opportunity to generate precursors having shorter chains and mostly trans-isomers, which could impart improved performance when the precursors are converted to downstream compositions (e.g., in surfactants). Formulators will benefit from identification of particular subclasses of surfactants that derive from renewable sources and have desirable attributes for glyphosate formulations.